Though the docs aren't indexed in the web search tool yet, JUNOS 5.5
adds the ability to perform loose uRPF now.
[edit int <name> unit 0 family inet]
set rpf-check mode loose
Watch for wrapping.
http://www.juniper.net/techpubs/software/junos/junos55/swconfig55-interf
aces/download/swconfig55-interfaces.pdf
Cheers,
-- steve